Fold the top edge of the curtain over and staple it in several locations to hold it in place. This will give more "meat" for the grommet to grip when you crimp it.
Then decide where the curtain will fold and crimp a small grommet into the fold as shown. The grommet should be able to punch through this - I've crimped grommets through fabric and it usually cuts a little hole for you. If not, an X-Acto blade should clean it up nicely.
Small tip: I made it so that the folds end up rubbing against the window - this makes it slightly annoying to push open the curtain completely; two hands are needed. See if you can fix my mistake on your design.